In the manufacture of many tape and label products in which a pressure sensitive adhesive is coated on a substrate such as paper or film, an essential component of the construction is the release coating. When a release coating is applied on the non-adhesive side of a backing, as in a roll of tape, it is commonly referred to as a "low adhesion backsize". The low adhesion backsize allows the roll of tape to be unwound with a reasonable amount of force (about 5 to 15 N/dm). On the other hand, a separate release coated sheet which is contacted with an adhesive coated side of a tape or label is referred to as a "liner", and in such a case, the peel force required to remove the tape or label is usually very low--typically less than about 1 to 2 N/dm. For this level of release, which is referred to as premium to easy, the usual coating material employed is some form of cured polysiloxane. These polysiloxanes are, for the most part, polydimethylsiloxanes substituted with a minimum number of various reactive functional groups. These polysiloxanes are coated on a substrate and subsequently chemically crosslinked. Ideally, these polysiloxanes should provide complete coverage of the substrate, cure completely to an ultra thin film (about 1 micron), have reasonable strength, be resistant to abrasion, and strongly adhere to the substrate. Other desirable features of such systems would include long term shelf stability and pot life prior to coating, followed by rapid cure on web, as well as the ability to easily adjust the release to higher levels than premium, if necessary. Few polysiloxane release coatings possess all of these optimal attributes.
Over the years, a number of polysiloxane materials having different curing chemistries were developed as release liner coatings for pressure sensitive adhesives. One of the first to find widespread acceptance was a composition comprising high molecular weight silanol terminal polydimethylsiloxane and a low molecular weight crosslinking copolymer of dimethyl and methyl siloxane. Cure was effected by heating the composition in the presence of organometallic catalysts. Although reliable premium release as provided, the polysiloxane coating suffered from a number of drawbacks; it was useful only on paper, it has to be applied from hydrocarbon solvents, and finally, the cure was extremely slow, often requiring days for completion.
More recently, several other polysiloxanes have become available, in which fast cure is triggered by exposure to ultraviolet radiation (UV). Such compositions and processes are exemplified by U.S. Pat. No. 4,563,539 (Gornowicz et al.); U.S. Pat. No. 4,359,369 (Takamizawa et al.); U.S. Pat. No. 4,554,339 (Hockemeyer et al.); and U.S. Pat. No. 4,597,987 (Hockemeyer et al.). These patents disclose oligomeric polydimethylsiloxanes substituted with pendant acryloxy alkyl groups which undergo rapid crosslinking using ultraviolet light sensitive photoinitiators in an inert atmosphere. In a similar fashion, U.S. Pat. No. 4,313,988 (Koshar et al.) and U.S. Pat. No. 4,822,687 (Kessel et al.) disclose polysiloxane compositions with pendant epoxy alkyl substituents which cure in seconds when super acids are generated from the UV catalyzed decomposition of certain onium salts. For these polysiloxane compositions, inerting of the coating is not necessary. In addition to the high speed of cure, these types of polysiloxane compositions offer the advantage of being completely stable before and during coating operations.
Another curing method which has proven very effective for the preparation of polysiloxane adhesives and sealants is the "moisture cure" of alkoxysilane functional polydiorganosiloxanes. In this system, atmospheric water is utilized to hydrolyze the alkoxy, especially trialkoxy, silane to --SiOH groups, which subsequently condense with other silanols or alkoxysilane groups to form --Si--O--Si--crosslinks. These reactions are catalyzed by certain tin compounds, such as dibutyl tin dilaurate. Although such materials have been reported useful as release coatings, as in, for example, U.S. Pat. No. 4,269,963 (Homan et al.), they, like the condensation cure silicones, are found to cure too slowly to be practical in most manufacturing operations.
Recently, however, Homan et al. (U.S. Pat. Nos. 4,743,474; 4,530,882; and 4,525,566) have disclosed moisture curable silicone compositions consisting of mixtures of predominantly polydiorganosiloxanes having two or more terminal and/or pendant trialkoxysilyl substituents and lesser amounts of tetraalkyl titanate esters which provide silicone release coatings that cure exceptionally rapidly under ambient conditions.
Although these compositions have indeed solved the problem of slow curing of silane substituted silicones, the actual use of such materials in practice is not without some formidable difficulties. These practical processing difficulties are well described by D. J. Huettner in a conference paper entitled "Moisture Curing Silicone Release Coating Technology: A Coating Process is the Missing Component", presented at the 1988 Pressure Sensitive Tape Council Technical Seminar. Huettner states that these silane/titanate mixtures are so extremely sensitive to ambient moisture that they cure instantaneously upon exposure to air, making them almost impossible to coat with ordinary equipment due to premature gelation on the substrate, in the coating equipment, and in the feed tanks.
Due to the limitations of known silicone release coating compositions, a need exists for a silicone release coating composition which has excellent shelf stability, processibility and pot life, and yet which cures rapidly and completely in atmospheric moisture in a controllable manner when coated on a variety of substrates. A need also exists for a release coating composition for which the level of release of the coating can be easily adjusted from premium to tight in a readily predictable and reliable manner. We have found such a release coating composition.